@MrsTerryPratchett has shown why your 5 yo is misbehaving and @KomodoOhno and others have noted that he shouldn’t be punished for being insecure and unhappy.
In short, the problem isn’t with your son, it’s with you and your partner.
How you and your partner are treating this child is cruel, heartless and not in his best interests. Things will get worse unless you make fundamental changes.
Firstly, I suggest that you and your partner start to practise positive reinforcement towards him. That means ignoring poor behaviour and instead look for anything he does that you can praise. Think of ways to vary this positive feedback. For example, say he’s very good at cleaning his teeth for a 5 year old. Or that he eats his breakfast every morning without making a fuss, and so on. Look to praise 3-5 things daily.
You and your spouse should say sorry to your son and that things are going to be different for him. Ask how he feels most of the time - using emojis if he can’t find the words. Find out from him what changes he’d like. It would be useful to have a chat with him each evening, maybe at bedtime, and find out how he’s doing. You’ve got a lot of bridges to build to help him trust you again.
You could also meet with his school/nursery to see how he is with them and explain why and how you intend to make improvements. Set up a regular meeting schedule with them.
Your older child will wonder what’s going on, so explain to them why and how you’re making changes.
By doing this, his behaviour should improve fairly soon but he’ll still have times when he reverts to old ways. Anticipate this and ignore it. Then talk to him about it during your daily 1:1. Tell him you understand why it’s happening and that it’s ok to do that. Give love, support, understanding and reassurance.
If ever a child deserved help, your son does so please work to make his life better for him.